Sorry, my fault. I change a sentence I wrote around, but didn't adjust the words. What I wanted to say was, NSTextView (and the abstract class NSText) both support setFont: in GNustep. For the class NSTextStorage this method is only provided as a scripting extension and currently GNUstep doesn't provide most of these extensions.

There wont be a problem to include this method if you really need it an provide an implementation. OK, this is really easy, here is one (code stolen from NSTextView), but why would we want it?

this is a class not existent on OpenStep, so it is more a Feature request.
Interestingly though NSTextView is a subclass of NSText, which had setFont even in OpenStep times and has it in GNUstep. So this exception is weird.